I. Field of the Invention:
This invention relates generally to apparatus for the bulk demagnetization of items exhibiting a remanent magnetic field, and more particularly an improved system for uniformly and efficiently demagnetizing items possessing a remanent magnetic field on a high volume, by subjecting the item to a demagnetizing field which rotates and which increases from essentially a zero amplitude to a predetermined maximum and then again decreases to a zero amplitude. This type of field profile is deemed ideal as far as its ability to leave the item in a completely demagnetized state.
II. Discussion of the prior Art:
In the Jackson et al U.S. Pat. 4,423,460, there is described an apparatus for the bulk erasing of magnetic recording media, such as magnetic recording tape used to store audio, video and digital information. It is the intent of that apparatus to leave the magnetic domains on the tape in a random orientation so that subsequent recording will take place on a "clean slate". As is pointed in the Jackson et al patent, noise is introduced into the recording process if the information signals to be recorded are imposed on a magnetic media that already has its domains uniformly oriented in given areas. The apparatus of the Jackson et al patent comprises a two piece, multi-legged iron core structure where each leg includes windings thereon with the diametrically opposing pairs of legs in each core being wound for opposite magnetic polarity. The windings are arranged to be energized from an alternating current 60 Hz supply through a phase shifting circuit so that, ideally, the current supplied to one pair of legs is 90.degree. out-of-phase with respect to the current applied to the windings on the other pair of legs. This results in the production of a rotating magnetic field in the gap between the apposed faces of the legs of the two cores. The windings are continuously energized by the AC source and the item to be demagnetized is made to pass, via a conveyor or the like, through the rotating magnetic field. According to the theory given in the Jackson et al patent, the field strength to which the item being demagnetized is subjected supposedly rises from zero as the item enters the field, reaches a maximum when the item is centered between the two cores and then decreases to zero as the item passes out of the influence of the magnetic field. In practice, however, a device built in accordance with the teachings of that patent will not yield the ideal demagnetizing field characteristic described above. The Jackson patent also suggests that a controlled variation in the demagnetizing field strength can be obtained by locating the item to be demagnetized between the apposed pole pieces and then moving those pole pieces apart to reduce the applied field toward zero. Because such operation necessarily varies the mutual inductance of the field coils, it will introduce unwanted variations in the uniformity of the rotating field.
It is well recognized that a rotating magnetic field can be produced by applying sinusoidal currents to two magnetic structures which are physically arranged to create mutually orthogonal magnetic field vectors when the AC currents are 90.degree. out-of-phase with respect to one another, e.g. induction electric motors. However, to produce a perfectly uniform rotating field, it is necessary that the magnitude of the currents be identical. Also, they must be supplied exactly 90.degree. out-of-phase with respect to one another. When a rotating field is to be used to demagnetize magnetic recording media, such as diskettes, magnetic tape cassettes, disk packs and the like, it is essential to proper demagnetization that the rotating field be perfectly uniform. Otherwise, non-uniformities in the material's erasure results.
The circuit of the aforereferenced Jackson et al patent utilizes capacitive reactance to produce the desired 90.degree. phase shift. The capacitor C.sub.1 in FIG. 4 is selected to produce a 45.degree. phase shift while capacitor C.sub.2 is selected to produce a 135.degree. sic (315.degree.) phase shift, resulting in the desired 90.degree. phase difference in the two coil system.
A reactive component will, of course, shift the phase between line voltage and line current by plus or minus 90.degree. depending upon the nature of the component (capacitor or inductor). A dissipative component (a resistor) does not shift the relative phase between line voltage and line current at all. The combination of a resistor and a reactive component will produce a phase shift between zero and plus or minus 90.degree.. The magnitude of the phase shift in this latter case is dependent not only on the value of the components, but on the frequency of the applied voltage. It is apparent, therefore, that the circuit of the Jackson patent relies upon resistive elements in the circuit, even though they are not explicitly shown. The uniformity of the magnetic field developed by the system of the Jackson patent is thus dependent upon the precision of the values of his reactive components and the stability of the line frequency. The incorporation of the dissipative elements, of course, also creates power loss in the form of heat. Short term heating effects and long term aging effects tend to change the values of the components employed, causing degradation of the symmetrical (circular) nature of the rotating demagnetizing field. As mentioned, variations in the line frequency will also cause the rotating demagnetizing field to deviate from circular.
The resistive elements employed in the phase shift circuitry of the Jackson et al patent are also wasteful of power and reduce the efficiency of the machine. Because the Jackson system utilizes ferrous metal cores in the demagnetizing assembly, this adversely impacts efficiency, versatility and the quality of the demagnetization process. Specifically, when ferrous materials are exposed to an AC magnetic field, eddy currents result and hysteresis losses occur which result in heating and a general loss of efficiency. Because ferrous materials exhibit a saturation point, they limit the magnitude of the field which may be generated. The ferrous core pieces of the Jackson patent also introduce hysteresis effects which tend to distort the sinusoidal magnetic field through the introduction of third harmonics. It is found that these third harmonics invariably detract from the quality of the erasure process.
A still further drawback of the system of the Jackson et al patent centers around the use of ferrous metal pole pieces which create areas of increased flux density proximate the poles. These nonuniformities, i.e., "hot spots", decrease the quality of the erasure and oftentimes leaves the magnetic domains in the material in other than a purely random state. For example, when an item, such as a reel of recording tape, to be demagnetized is made to pass between the apposed core pieces upon which the field windings are arranged, as it passes between the trailing pole pieces, it will not be subject to a rotating field, but only to a strong sinusoidal magnetic field because the coil pair producing the quadrature field is now at a considerable distance from the item being treated. As a result, the item being erased is actually subject to a remagnetizing field upon exit from the system.
The system of the present invention obviates all of the foregoing deficiencies of the prior art as represented by the Jackson et al patent. The demagnetization system hereindescribed is virtually impervious to variations in line frequency since the energy used in the demagnetization process is first stored as a DC voltage and then converted through a microprocessor-controlled inverter circuit to an AC current of a precise frequency, independent of the frequency supplied by the power company. The microprocessor used in the system of the present invention monitors the operation of the system to insure a 90.degree. phase shift between the energizing currents flowing through the orthogonal field windings. Thus, the system is immune from loss of uniformity in the rotating demagnetizing field vector due to component value drift and ferrous metal introduction. Also, as will become apparent as the description of the preferred embodiment is presented, the demagnetization system of the present invention uses no dissipative elements (resistors) in the high power section, resulting in extremely high efficiency and low power consumption. Furthermore, the system of the present invention can operate at any level of magnetizing force because the field is not limited by the saturation characteristics of the particular iron alloy used in the fabrication of pole pieces as in prior art systems. It may operate at any frequency or any phase shift between coils and on any size items to be demagnetized or any field strength, subject only to the state-of-the-art of power switching technology.
Because the system of the present invention operates at a frequency considerably higher than the 60 Hz current supplied by the power company, the demagnetization process can take place at a substantially greater rate than systems which merely utilize line power, thus increasing the processing rate of the equipment.